(1) Field of the Invention
The present invention relates to an image formation apparatus including a fixing device, and in particular to technology for, when an image is thermally fixed on a smaller-sized recording sheet among several-sized recording sheets by using an induction heating method, suppressing a temperature increase in an area corresponding to a portion that is out of contact with the sheet when the sheet passes through a fixing nip thereof.
(2) Description of the Related Art
In recent years, some fixing devices included in electrophotographic type image formation apparatuses or electrostatic recording type image formation apparatuses are starting to utilize heat sources of an induction heating type that are compact and have relatively high heat conversion efficiency. Such fixing devices are gaining popularity for conserving energy, saving space, reducing a warm-up period, and so on.
In particular, it is possible to manufacture a heat generation member with a significantly small heat capacity by configuring a fixing device to direct magnetic flux, which is generated by supplying an alternating electric current to excitation coil, to a conductive heat generation layer provided at a heat generation member being out of contact with core components, with use of the core components such as ferrite cores, such that the heat generation member is locally heated. This can reduce the warm-up period to a great extent.
However, with the heat generation member having a small heat capacity, the heat cannot be easily transmitted. Here, if small-sized recording sheets having a small width are continuously used, the temperature of portions of the heat generation member that do not come into contact with the recording sheets (hereinafter, “contactless portions”) will be abnormally increased, the contactless portions being outer edges of the heat generation member in the width direction thereof. This may thermally damage or deteriorate the components positioned in the vicinity of the contactless portions. Also, if a recording sheet having a large width is used immediately after the small-sized recording sheets, then the hot offset will occur only in the outer edges of the printed sheet in the width direction thereof, and the printed recording sheet will have uneven glossiness.
There are several techniques that are known to suppress the temperature increase in the contactless portions in the above-described fixing device, e.g., a technique to shield only a part of the magnetic flux that is proceeding toward the contactless portions by moving a conductor component in accordance with the sheet width, and a technique to cancel out only a part of the magnetic flux that is proceeding toward the contactless portions by using degaussing coils.
The following documents disclose techniques to suppress an excessive increase in the temperature of the contactless portions by providing the heat generation member for the above-described fixing device with a magnetic shunt alloy whose Curie point is somewhat higher than the fixing temperature. Due to the presence of such a magnetic shunt alloy, when the temperature of the contactless portions has been increased to some extent, the heat generation member has the self-temperature control function of automatically losing magnetism in the contactless portions, and reducing the heat capacity.
Patent Document 1 (Japanese Patent Publication No. 3988251) discloses, for example, an image heating device that causes a heat generation layer including a magnetic shunt alloy layer to come into contact with the back of a fixing belt. According to Patent Document 1, the fixing belt has a very small heat capacity, rendering a warm-up period extremely short. Furthermore, it is disclosed in Patent Document 1 that, as the self-temperature control is effectively performed, excitation components would not get thermally damaged even when printing on a succession of recording sheets each having a small width.
Patent Document 2 (Japanese Laid-Open Patent Application No. 2007-156065) discloses a fixing device having a shield plate provided inside a cylindrical heat generation roller made of a magnetic shunt alloy and extending in an axial direction of the heat generation roller. The shield plate has a cross section substantially in a C shape taken perpendicularly to the axial direction such that an edge of each end portion of the shield plate is opposed to the heat generation roller. According to Patent Document 2, the thermal load in relation to the heat generation roller is small. This, Patent Document 2 discloses, suppresses an excessive temperature increase, reduces a warm-up period, and prevents occurrence of an offset, thus providing a high-quality fixing performance.
Patent Document 3 (Japanese Laid-Open Patent Application No. 2007-264421) discloses the following fixer. The fixer includes a low-resistance, plate-like member extending in the axial direction thereof and provided inside a cylindrical fixing rotary body. The low-resistance, plate-like member has a central portion facing a central portion of the fixing rotary body. The central portion of the low-resistance, plate-like member is thinner than each end portion of the shield plate, wherein each end portion has a larger thickness than the thickness by which magnetic flux penetrates thereinto. According to Patent Document 3, the self-temperature control function works effectively especially on both end portions of the fixing rotary body in the width direction thereof, and suppresses an excessive temperature increase in said end portions without lowering the warm-up performance and heating efficiency of the central portion of the fixing rotary body. Patent Document 3 discloses that, even when printing on a succession of small-sized recording sheets, it is possible to reliably suppresses the excessive temperature increase in said end portions without the central portion of the fixing rotary body being insufficiently heated.
Meanwhile, during a stand-by period in which the image formation is not performed, the temperature of the fixer needs to be maintained at a stand-by temperature from which the fixer can reach the fixing temperature in a few seconds, so as to promptly start the fixing whenever an instruction to perform the image formation is issued. Especially, if the fixer has a poor warm-up property, the stand-by temperature must be set high. This is not suitable for saving energy. Furthermore, if the fixer has a poor warm-up property, the user has to wait for quite a while upon turning on the power of the image formation apparatus, which is not favorable.
In order for the fixing device to save energy and have a good warm-up property, it is effective to downsize the structure thereof and thereby reducing the heat capacity thereof. Accordingly, the fixing device is desired to be more compact.